This invention relates to pressure vessels and their manufacture, and more particularly to filament-wound plastic pressure vessels having side tap openings cut through their walls.
Filament-wound plastic pressure vessels are widely employed in applications requiring superior strength-to-weight characteristics, chemical resistance, and predictable stress characteristics in specific directions. Strands or rovings of fibers, particularly continuous glass fibers, are pre-impregnated with a suitable thermosetting resin such as polyester or epoxy resin, and wound about a mandrel under controlled tension and in a predetermined pattern. The mandrel may be of a flexible or destructible material capable of being removed from the winding after curing, or may be a permanent structure remaining in the finished article. In the latter arrangement, the mandrel comprises a liner which may be a blow-molded or rotationally cast thermoplastic compatible with the thermosetting resin carried by the filaments and with the fluid which it is intended to contain. Thus, the liner serves as a fluidtight and chemically resistant barrier, while the filament winding provides structural reinforcement thereto.
Filamant-wound lined pressure vessels are employed in a wide variety of end-use applications, such as pressure accumulator tanks in home water systems and in filter tanks for swimming pools and the like. In such applications, it is desirable, and often necessary, to provide access openings in the side wall of the filament-wound, cylindrical pressure vessel for fittings such as distributor tubes, plumbing connections, or valve connections. In the case of pressure vessels fabricated by laying up porous, fibrous mats or providing a fibrous preform in the shape of the container or tank to be produced and impregnating the mats or preforms with a settable resin, the problem of providing an additional opening in the side wall of the tank is solved merely by thickening the side wall about the periphery of the additional opening to provide reinforcement in this region and to provide an adequate depth of opening for tapping to receive a connecting threaded conduit. According to U.S. Pat. Nos. 3,137,898 and Re. 25,241, the additional thickness is provided by laying up preformed pads in the desired area prior to resin-impregnation.
More complex problems, however, are present in the provision of access openings in filament-wound tanks. An obvious solution to the problem is to cut access openings in the liner which is to serve as the winding mandrel and then plug those openings with temporary closure members which serve to direct the winding filaments around the access openings. After curing, the temporary plugs are removed and permanent fittings are supplied. However, as may be appreciated, the provision of the temporary plugs disrupts the desired winding pattern and greatly increases the filament bulk around the access openings.
Prior art efforts to provide access openings in filament-wound pressure vessels also included the step of merely cutting through the side wall of the completed filament-wound tank. This, of course, was soon recognized as an unacceptable solution to the problem, since the load bearing filaments were severed, thus destroying the structural integrity of the vessel.
More acceptable solutions may be found in U.S. Pat. Nos. 3,106,940; 3,112,234; and 3,293,860. According to those patents, reinforcing patches are wound into the side wall of the tank. A number of patches may be superposed by applying additional patches between winding passes so that a stacked array of reinforcing material is provided in the side wall. After curing, the access opening is cut through the reinforced area. In all of the aforementioned patents, however, the patch is formed by peripherally winding continuous filaments between closely spaced plates, much like one would wind a spool of thread to provide a spiral-wound, flat washer-like structure having a central opening conforming to the size of the opening to be provided in the side wall of the vessel. While the spiral-wound reinforcing patch may provide dimensional stability to the tapped opening upon pressurization of the vessel, the spiral-wound filaments do not provide adequate stress paths between the severed ends of the side wall winding and around the cut opening causing the severance. A helical or spiral reinforcing patch does not provide a natural stress path flow around the opening, but intersects the cut filaments abruptly and in a non-tangential fashion. Thus, while the reinforcing patch promotes dimensional stability to the opening upon pressurization of the tank, no such stability is afforded the cut filaments by the patch.
A further deficiency in the provision of a peripherally or helically wound patch is that the central opening of the patch must be precisely dimensioned to the size of the intended cut opening in the tank. If the aperture in the patch is too large, unreinforced portions of the cut opening will provide stress concentration points which may cause cracking between the opening and the reinforcement, and if the opening is too small for the intended aperture, the peripheral windings of the patch will be severed by the cutting operation. Moreover, the final cutting operation must precisely conform to the location of the opening in the patch and this operation is difficult, since the patch is covered by windings and by resin.